1. Field of the Invention
The present invention relates generally to a cooled deformable mirror structure and to a method for cooling a deformable mirror for use as a wavefront phase modulator in optical systems having lasers which generate high energy flux levels. Such systems commonly utilize deformable mirrors to reflect a laser beam in a desired direction and/or to correct the laser beam for wavefront distortions, or to encode the wavefront by introducing known distortions thereto. Because a mirror for use in such applications cannot be constructed to reflect the totality of laser radiation incident thereon, the small portion that is absorbed by the mirror surface can cause thermal overloading of the mirror surface and its supporting structure due to the flux level of the incident laser beam. The absorbed thermal energy results in stresses in the mirror's support structure which can result in unwanted distortion of the surface of the mirror, or the destruction of the mirror surface and its supporting structure.
2. Description of the Prior Art
The use of deformable mirrors for the correction of wavefront phase distortion and for encoding wavefronts by introducing known distortions in a wavefront is known in the prior art. See, for example, U.S. Pat. No. 3,904,274 which issued on Sept. 9, 1975 and U.S. Pat. No. 4,257,686 which issued on Mar. 24, 1981, both of which are incorporated herein by reference. The use of a fluid for cooling mirrors used in laser applications to reduce thermal loads imposed by the laser beam is also known in the art. An example of such a mirror is disclosed in U.S. Pat. No. 3,909,118 which issued on Sept. 30, 1975. Moreover, the use of fluid cooling in deformable mirrors is also known in the prior art as exemplified by U.S. Pat. No. 4,143,946 which issued on Mar. 13, 1979; U.S. Pat. No. 4,202,605 which issued on May 13, 1980; and U.S. Pat. No. 4,239,343 which issued on Dec. 16, 1980.
U.S. Pat. No. 4,143,946 is of interest for its disclosure of a deformable mirror which uses electromagnetic actuators to control the distortion of a continuous faceplate. The coolant is allowed to spread over the rear side of the faceplate after passing through a plurality of nozzles. The extent to which the rear side of the faceplate is cooled is determined by the size of the nozzles, their distance from the faceplate, and the pressure of the cooling fluid.
U.S. Pat. No. 4,202,605 is of interest for its disclosures of a cooled segmented mirror which uses piezo ceramic actuators to dynamically position discrete elements of a mirror. The mirror is constructed from a plurality of discrete mirror elements, each of which is individually positioned by its own piezo ceramic actuator. Coolant is led toward and away from each mirror element through the element's associated actuator. Each actuator is connected via two coolant supply lines to a distribution manifold which distributes and collects the cooling fluid.
U.S. Pat. No. 4,239,343 is of interest for its disclosure of a deformable mirror which includes a conduit composed of thin-wall copper tubing through which coolant may be circulated to cool the deformable mirror. The tubing traverses spaces above a series of spherical piezoelectric actuators which are used to induce localized variations in the shape of the surface of a mirror which is supported by the actuators.
It is known that coolant introduced against the rear surface of a mirror faceplate of a deformable mirror at an excessive pressure will cause vibration and distortion in the mirror structure, which may introduce unacceptable distortions into the laser wavefront reflected from the mirror. However, the pressure at which coolant must be introduced to cool the mirror structure is dependent on both the cross-sectional area and length of the path that the coolant must follow in traveling through the mirror structure, and is also directly related to the quantity of thermal energy that the coolant must absorb. If coolant is circulated through the mirror at a low pressure in an effort to reduce pressure-induced vibration and distortion, the rate at which the coolant is circulated may be insufficient to permit the coolant to absorb a sufficient quantity of thermal energy to prevent excessive heating of the surface of the mirror.
Prior-known cooled deformable mirrors failed to provide a continuous deformable mirror surface in which coolant operating at a low pressure cooled the mirror base and actuators and then traveled only a short distance while cooling the surface of the faceplate. The present invention overcomes this defect by disclosing a deformable mirror apparatus and a method for cooling a deformable mirror in which the mirror's continuous mirror surface is cooled by a coolant operating at a sufficiently low pressure to prevent vibration and distortion induced by the coolant's flow from distorting the laser signal reflected by the mirror. The low coolant pressure is made possible by the short distance that the coolant is required to traverse across the faceplate containing the mirror's reflecting surface.